1. Field of the Invention
The present invention relates to a control method for a continuous variable valve lift (CVVL) engine. More particularly, the present invention relates to a control method for a CVVL engine for preventing a knocking phenomenon in an idle state of the engine.
2. Description of Related Art
An internal combustion engine combusts mixed gas in which fuel and air are mixed at a predetermined ratio through a set ignition mode to generate power by using explosion pressure.
In this case, a cam shaft is driven by a timing belt connected with a crank shaft that converts linear motion of a cylinder by the explosion pressure into rotating motion to actuate an intake valve and an exhaust valve, and while the intake valve is opened, air is suctioned to a combustion chamber and while an exhaust valve is opened, gas which is combusted in the combustion chamber is exhausted.
In the operations of the intake valve and the exhaust valve, only when a valve lift and a valve opening/closing time (timing) are controlled according to a rotational speed of an engine, optimal engine performance is secured.
As such, a CVVL device has been developed and applied, in which a plurality of cams that drives the intake valve and the exhaust valve is designed or the intake valve and the exhaust valve are implemented to operate by different valve lifts according to an engine rpm in order to appropriately implement the operations of the intake valve and the exhaust valve according to the rotational speed of the engine.
Meanwhile, when the engine tips in an idle state, a compression ratio is increased and self-ignition occurs due to a sudden increase of an air quantity, and as a result, a knocking phenomenon occurs.
In order to prevent the phenomenon, two methods can be generally used in a vehicle adopting the CVVL device, and one therebetween is to decrease a valid compression ratio by applying an intake cam at a parking position and high-lifting the valve lift and the other one is to decrease the valid compression ratio by maximally advancing the intake cam and low-lifting the valve lift. The second method to maximally advance the intake cam and low-lift the valve lift is more advantageous than the first method in terms of fuel efficiency therebetween.
In the related art, the knocking phenomenon was prevented by decreasing the valid compression ratio by designating one method between the two methods, but when the engine is in the idle state, advancing of the intake cam may be impossible due to insufficient oil pressure. However, in the related art, since a countermeasure considering the case in which the cam advancing is impossible was not provided, appropriate action cannot be taken, and as a result, the knocking phenomenon occurred or the fuel efficiency deteriorated.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.